How get length of Navigation Mesh?
Since each of the small planes are the faces of the mesh, iterating through the polygons on the object mesh data provides access to the coordinates of the center of the face. Then using two consecutive coordinates, the distance between the centers can be calculated. This approach assumes the previous face is adjacent to the current face. Additionally, because accessing mesh data uses bpy, any changes will not occur while the game engine is running.

import bpy, mathutils

plane = bpy.data.objects["Plane"]

previous = None

for poly in plane.data.polygons:
    center = mathutils.Vector(poly.center)

    if previous:
        previous -= center
        #print length and center
        print(previous.length, center)

    previous = center